1. Field of the Invention
The present invention relates to a surface renewable hydrogen ion electrode made of iridium oxide-glass or ceramic composite materials, and, more particularly, to a surface renewable iridium oxide-glass or ceramic composite hydrogen ion electrode, which has long lifetime due to its excellent physical strength, pH dependency approximate to the theoretical value (59 mV/pH unit), and high surface renewability, and to a method of manufacturing the same.
2. Description of the Related Art
Generally, pH is one of basic analytical parameter measured in various fields, such as environmental, nutritional, medical, medicinal, physiological, or chemical fields. In particular, pH glass membrane electrodes have been used generally in the case where electrodes are exposed to the possibilities of contamination due to prolonged exposure, as in a long-term water-quality monitoring system, or in the case where pH is measured in harsh environments causing contamination of the surface of the sensor, like samples used in soil analysis processes, food production processes and the like. However, the pH glass membrane electrodes are fragile, so they can be broken easily. They have to be changed or reactivated periodically whenever they are contaminated or fouled due to the long-period exposure of the sensors to environmental samples. However, it is hardly to find any effective method for overcoming the inactivation and fouling problems.
Polymer membrane ion selective electrodes or coated-wire type selective electrodes, based on ionophores or ion carriers, are advantageous in that they are easily manufactured and can be miniaturized in a small size. But they are disadvantageous in that, since they are formed of a physically weak and thin polymer film. They can be easily torn and it is impossible to reactivate and regenerate the surface thereof.
Meanwhile, it was reported in the document (A. Fog, R. P. Buck, Sensors and Actuators, 1984, 5, 137-146) that metal oxide films formed on the surface of precious metal electrodes can be used for measuring hydrogen ion concentration because the electrode potentials due to the oxidation-reduction reaction of the metal oxides are dependent on the hydrogen ion concentration, and these metal oxides include TiO2, RuO2, RhO2, SnO2, Ta2O5, OsO2, PdO2, PtO2, IrO2, and the like. The hydrogen ion selective electrodes using metal oxides are mostly based on the fact that the potentials due to the reversible oxidation-reduction reactions of the metal oxides are dependent on the hydrogen ion concentration. These metal oxide film based pH electrodes are manufactured using a sputtering method, a pyrolysis method, or an electrochemical oxidation method on metal foils or wires. As the result of basic electrochemical research, it was found that, among these metal oxides, iridium oxide (IrO2) exhibits the most excellent hydrogen ion selectivity and sensitivity.
A typical electrochemical oxidation method is a method of dipping iridium (Ir) electrodes in a sulfuric acid solution and oxidizing iridium of the electrodes to iridium oxides by cycling the potential of the electrode in the range of −0.25 to +1.25 V (vs. SCE). The iridium oxides formed on the metal electrodes are hydrated iridium oxides, such as IrO2.4H2O, Ir(OH)40.2H2O, [IrO2(OH2.2H2O]2−, or the like. It is disclosed in the document (D. Midgly, Talanta, 1990, 37, 767-781) that the iridium oxides exhibit super-Nerntian response of about 90 mV/pH unit according to Equation 1 below.2[IrO2(OH)2.2H2O]2−+3H++2e−[Ir2O3(OH)3.3H2O]3−+3H2O  [Equation 1]
Further, in the case of iridium oxide film electrodes formed on the surface of platinum or iridium electrodes using a sputtering method or a pyrolysis method. The iridium oxides formed on the electrodes are anhydrous iridium oxides, and exhibit responses of about 59 mV/pH unit according to Equation 2 or 3 below. It is disclosed in the document (P. VanHoudt, Z. Lewandowski, B. Little, Biotech. Bioeng. 1992, 40, 601-608).IrO2+H++e−IrO.OH  [Equation 2]or2IrO2+2H++2e−Ir2O3+H2O  [Equation 3]
Recently, novel hydrogen ion electrodes having hydrogen ion sensitivity, durability and surface renewability were developed by combining the characteristics of above iridium oxide film hydrogen ion electrodes, having durability and hydrogen ion concentration dependency, with technologies for manufacturing carbon-polymer composite electrodes (H. Quan, W. Kim, K.-C. Chung, J. Park, Bull. Korean Chem. Soc. 2005, 26, 1565-1568).
The iridium oxide/carbon-polymer composite hydrogen ion electrodes have an advantage in that the medium thereof is composed of polymer materials and carbon black particles or graphite particles, which are conductors, and uniformly includes iridium oxide particles, exhibiting selective sensitivity to hydrogen ion, so that the electrodes have hydrogen ion selectivity and physical stability due to the mechanical strength of the polymers, thereby easily obtaining a renewable electrode surface through a simple polishing process, whenever the electrodes are inactivated and contaminated.
However, the iridium oxide/carbon-polymer composite pH electrodes have problems in that, although the electrodes have improved physical stability and surface renewability compared to conventional glass electrodes or polymer film electrodes, the manufacturing method of the electrode is complicated, the pH dependency of the electrodes varies greatly depending on the electrodes, and hysteresis occurs during repeated use of the electrodes. Moreover, the iridium oxide/carbon-polymer composite pH electrodes have a problem in that surface sensing materials can be worn out easily due to the low hardness of the polymeric media at the time of surface grinding and polishing for surface regeneration.